[TowerTalk] ground and conductivity

[Jim Lux]

At 02:34 PM 2/28/2005, Jerry Keller - K3BZ wrote:
>Is there any way I can take meaningful soil conductivity readings with 
>simple equipment?
>
>73,  Jerry K3BZ


Depends on what you mean by simple, and what sort of spatial resolution you 
want.

A straightforward 2 or 4 probe resistance measurement at 60Hz will get you 
in the ballpark (within a factor of 10, and probably closer). Don't put the 
probes too far apart, because it basically measures the resistance at a 
depth of half the spacing of the probes, and skin depth at HF ranges around 
1-15 meters (based on Hagn's ACES paper in 1988, the plots of which are in 
the ARRL antenna book)

The ARRL antenna book describes a technique developed by George Hagn at SRI 
where you basically create a parallel conductor open wire line inserted 
into the soil and measure the transmission line's impedance.  You'll need 
some sort of impedance bridge for this measurement.  Typial measurements 
from his paper for 6 inch long probes 3" apart were a few hundred ohms 
magnitude and -40 degrees phase  (like 230-190j ohms).  Perhaps a suitable 
6:1 transformer/balun and a MFJ/Autek analyzer might work?

This technique has seen quite a bit of rigorous validation.

Another approach is to use a dipole close to the ground and measure it's 
feedpoint impedance.  The tradeoff here is the closer to the ground, the 
more sensitive it is to being exactly the same distance everywhere, the 
physical size, etc.  The original NEC validation testing used this kind of 
approach.

Both of these approaches should give you values good to perhaps 25%.


In theory, one should be able to measure the Q of a small resonant loop 
close to the surface, but calibration might prove to be tricky, as would 
accurately de-embedding the other loss sources in a relatively high Q system.